1. Field of the Invention
The present invention relates to a display device, and in particular, a thin display device, such as a liquid crystal display or an organic EL display, having a curved display screen.
2. Description of the Related Art
In terms of reduction in thickness and weight of a display device, a liquid crystal display is increasingly demanded in a computer display, a terminal for a mobile phone, and a TV. The liquid crystal display has a flat screen.
The reduction in thickness ensures the liquid crystal display to be installed at various places. A place where the liquid crystal device is installed is not limited to a place having a flat surface, but there is a need to install the liquid crystal device at a place having a curved surface. The places having a curved surface include, for example, columns, and walls of streetcars or buses. Meanwhile, there are apparatus in which a display itself needs to be curved. For example, a Japanese pinball game machine (pachinko), a slot machine, and a game machine at a game center may be exemplified. If the liquid crystal device is used in such a game machine, the liquid crystal display needs to be curved. In regards to mobile phones that are increasingly demanded, for improvement in visibility, a display unit having a curved surface is demanded.
However, it is very difficult to manufacture a liquid crystal display panel by using a curved glass substrate. Accordingly, a flat liquid crystal display panel is manufactured by using a flat glass substrate, and thereafter, the liquid crystal display panel is curved. It is necessary to contrive a way to uniformly curve the flat liquid crystal display panel.
JP-A-8-286174 discloses a technology that forms a liquid crystal display panel having a curved surface using a phenomenon that a polarizing plate is curved toward an absorption axis according to temperature and humidity. That is, if the polarizing plate is attached to a liquid crystal cell formed by a TFT substrate and a color filter substrate, and is then left in a temperature-controlled bath at specific temperature and humidity for a predetermined time, thereby a liquid crystal display panel having a specific curved surface is formed.
JP-A-2006-106079 discloses a technology that, when a flat liquid crystal display panel is curved, attaches a uniaxially extended film onto a concave surface side, and leaves the liquid crystal display panel under an atmosphere at 40° C. to 200° C. for 10 minutes to 24 hours, thereby curving the liquid crystal display panel. That is, the liquid crystal display panel is curved by using a phenomenon that the uniaxially extended film is contracted in the extension direction.
The liquid crystal display panel is formed by adhering polarizing plates on the front and rear surfaces of the liquid crystal cell. The liquid crystal cell has a TFT substrate, on which thin film transistors (TFTs) and pixel electrodes are formed, and a color filter substrate, on which color filters are formed, with liquid crystal interposed therebetween. A liquid crystal display panel with a curved surface is formed by curving the liquid crystal cell and the polarizing plates at a time.
In general, the polarizing plate is formed by extending a PVA film in one direction and dip-coating iodine. At this time, the iodine particles are arranged in the extension direction, and the extension direction becomes an absorption axis. For this reason, in the liquid crystal display panel, if tension continues to be applied in a direction perpendicular to the extension direction, that is, the absorption axis, polarization characteristics may be deteriorated. Meanwhile, a retardation film is used in the liquid crystal display panel, but similarly to the polarizing plate, the retardation film is uniaxially extended. For this reason, similarly to the polarizing plate, if tension continues to be applied in the direction perpendicular to the extension axis, that is, the absorption axis, optical characteristics may be deteriorated. In case of a curved liquid crystal display panel, since normal stress is applied to an optical film at its surface, the characteristics of the optical film may be deteriorated, and display characteristics, such as contrast, may be deteriorated.
According to the technology disclosed in JP-A-8-286174 JP-A-2006-106079, the liquid crystal display panel is curved by using a phenomenon that a uniaxially extended film is likely to be curved in a specific direction. However, these documents have not suggested and described the above-described problem, that is, if tension continues to be applied in the direction perpendicular to the extension direction of the polarizing plate or the like, that is, the absorption axis, optical characteristics may be deteriorated.